This invention concerns a process for producing a gasoline fraction of high octane number and low to nil content of olefinic hydrocarbon.
The pyrolysis processes, for example, steam-cracking, catalytic cracking and coking are among the most important processes for producing gasoline. By way of example, steam-cracking of hydrocarbons provides, in addition to ethylene, various products including a C.sub.5 or C.sub.5.sup.+ fraction, for example, a C.sub.5.sup.+ fraction having a final boiling point between 150.degree. and 220.degree. C.
According to a known treatment, the most unsaturated hydrocarbons of these fractions, such as the diolefinic hydrocarbons and the alkenylaromatic hydrocarbons are hydrogenated without substantial conversion of the mono-olefinic and alkylaromatic hydrocarbons to saturated hydrocarbons. In certain cases, only the C.sub.5 fraction is treated, the C.sub.6 -C.sub.8 hydrocarbons being previously separated and reserved to the production of aromatic hydrocarbons; in other cases, the whole C.sub.5.sup.+ cut is subjected to hydrogenation.
Gasolines of high mono-olefins content are thus obtained, the octane number of which is not fully satisfactory since, although the research octane number (F.sub.1) is high, the motor octane number (F.sub.2) is low. This is particularly the case for of the C.sub.5 fractions whose clear F.sub.1 is about 95 but clear F.sub.2 is only about 80.
Another source consists of catalytic cracking gasolines, for example fluid bed catalytic cracking gasoline (FCC). This gasoline of high mono-olefins content is practically free of diolefins and alkenylaromatic hydrocarbons; it can be used as such without further hydrogenation as applied to steam-cracking gasolines.
It is also known that the C.sub.5 fractions containing monoolefins can be modified by reaction with an alcohol in the presence of an acid catalyst: the C.sub.5 mono-olefins having a tertiary carbon atom as part of the unsaturated double bond, i.e. 2-methyl-1-butene and 2-methyl-2-butene, react rather selectively to yield an ether. This operation has a double advantage: the motor octane number (MON) increases as the result of the conversion of low MON olefins to high MON ether; on the other hand, due to a decrease of the olefins content, the lead susceptibility increases, which is advantageous since, in many countries, a reduction of the lead additive content of gasoline is required.
It appears that, although the Research octane number and the Motor octane number are routinely determined, the Motor octane number is closer to the truth and conforms better to the present motor requirements. Thus, in the German Federal Republic, a new rule imposes a minimum motor octane number in addition to the required research octane number. Also in the USA, the average value of the octane numbers: EQU (F.sub.1 +F.sub.2)/2
is taken into consideration.
The modification of a gasoline or of a C.sub.5 fraction by conversion of 2-methyl-1-butene (M2B1) and 2-methyl-2-butene (M2B2) to methyl tert.amyl ether has already been proposed, for example in the British Pat. No. 1 176 620, the U.S. Pat. No. 3 482 952 and the French Pat. No. 2 411 881. The latter patent suggests applying the process to a partially hydrogenated gasoline fraction, obtained by steam-cracking of naphtha or heavier hydrocarbon cuts: this partially hydrogenated gasoline fraction is distilled to yield a C.sub.5 cut which is thereafter reacted with methanol.
The partial hydrogenation, also called selective hydrogenation, of a steam-cracking C.sub.5 cut (or of gasoline comprising this cut) results in the conversion of at least 90% of the C.sub.5 aliphatic and cycloaliphatic diolefins to mono-olefins, the conversion of the mono-olefins (initial+formed from the diolefins) being itself low or nil, for example about 10 to 20% at the most.
It results therefrom that, when esterifying this cut with an alcohol, there is obtained a C.sub.5 cut comprising an ether (tert.amyl methyl ether or TAME when starting from methanol) and a high proportion of mono-olefins, essentially n-monoolefins and cyclomono-olefins which have not reacted with the alcohol.